The directional coupler is widely used for a RF device. For instance, the directional coupler is disposed between an amplifier and an antenna. The directional coupler is able to monitor and measure the RF signal transmitted by the RF device, such as power of transmission or reflection of the RF signal, to adjust the output power of the RF signal according to the measurement result.
Generally, a power detector is connected to the directional coupler to detect a coupling signal of the directional coupler and calculate power of the RF signal passing through the directional coupler. However, the coupling factor and the insertion loss of the directional coupler depend on the frequency band of the RF signal. For instance, the coupling factor and the insertion loss of the directional coupler may increase at high frequency band to result in failure of the SPEC of the power detector.